During editing, there are many situations where one would like to do a command based on the word under cursor. Idea is to yank into a register and then use this register automatically. Basis of this functionality are two commands

Yanking into a register, for example: "zyiw - will yank the word into z reg, "byy - yanks the line into b reg

Concatenating the register to a command and executing it. :exe "/".@z.""<CR> - searches for string in z reg

Alternately you can replace @z by <C-r>z which will directly replace z by its content.

Some more examples:

a) Let's say that you have a file that has a list of files. You don't want to edit all the files, but selectively.

:map <F2> "zyiw:exe "vs ".@z.""<CR>

Pressing <F2> will open the file in vertically split window.

b) Browsing a text file and saving all those difficult words you want to refer to during leisure